1. Field of the Invention
The present invention relates to a method for acquiring audio signals and an audio acquisition system capable of implementing said method.
In the television and movie fields and the like, there is an increasing need to record sounds accurately in the three-dimensional environment in which shooting is taking place, so that they can be reproduced faithfully at the user's premises.
Recording sounds in a three-dimensional environment involves the necessity of knowing the pressure and speed of the air particles in a certain spatial point.
To this end, it is currently known to use microphone probes which comprise multiple microphone capsules arranged on a surface, e.g. a spherical surface.
One example of such probes is the microphone probe available on the market under the name “EigenMike32” and manufactured by the American company “mhAcoustics”.
FIG. 1 shows an example of a probe 11 which allows audio signals to be acquired from multiple spatial directions. Said probe 11 comprises a number Y (in this case thirty-two) of microphone capsules B arranged on a rigid and substantially spherical shell C.
Each of the capsules B detects one audio signal coming from a different spatial direction.
By appropriately combining these signals it is possible to obtain a signal corresponding to the signal that would be measured by a microphone having certain desired characteristics.
Thanks to these probes, the user can use “virtual” microphones having the desired characteristics of directivity (cardioid, supercardioid or the like) and position (azimuth, elevation, etc.).
2. Present State of the Art
Probes of this type are generally used in combination with graphic systems in order to display for the user any noise sources and identify any mechanical defects in a machine (e.g. a broken tooth of a toothed wheel) or any sources of noise pollution.
For this purpose, much importance is attributed in the known probes to the microphone directivity, and much effort is being made to define optimal filters which can ensure the best possible directionality.
Once the optimal theoretical filters have been identified, the audio signal of the virtual microphone required by the user is generated by appropriately weighing the filter outputs and by applying thereto delays and gains which are suitably calculated and then combined together in order to obtain certain forms of microphone directivity.
A first limit of these probes is related to the fact that the use of predetermined theoretical filters, although it provides good directivity, often does not ensure a good audio signal quality.
Moreover, another limit of these known probes is the fact that they can only provide good directivity up to certain frequencies, typically around 4 kHz, whereas beyond which the directivity tends to deteriorate.
These probes are therefore not suitable for use in the television or cinematographic environment, wherein, in addition to the microphone directionality, it is also very important to be able to acquire high-quality audio signals.